King Boo's Crown
From the Super Mario Wiki
King Boo's Crown is the final piece of jewelry Luigi collects before he saves Mario in Luigi's Mansion. In Luigi's Mansion, the crown holds a red diamond at the top. Officially, King Boo's crown also counts as a Red Diamond. The crown is worth the same amount as a single coin.
In Luigi's Mansion: Dark Moon, however, King Boo appears to sport the crown's initial appearance, except the gem, which is now a stone that resembles an amethyst, matching King Boo's color theme. This gem gives him enough power to shatter the Dark Moon.